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physics
college physics 2nd
College Physics 2nd Edition OpenStax - Solutions
How is heat transfer related to temperature?
Describe a situation in which heat transfer occurs. What are the resulting forms of energy?
When heat transfers into a system, is the energy stored as heat? Explain briefly.
What three factors affect the heat transfer that is necessary to change an object’s temperature?
The brakes in a car increase in temperature by ΔT when bringing the car to rest from a speed v. How much greater would ΔT be if the car initially had twice the speed? You may assume the car to stop sufficiently fast so that no heat transfers out of the brakes.
Heat transfer can cause temperature and phase changes. What else can cause these changes?
How does the latent heat of fusion of water help slow the decrease of air temperatures, perhaps preventing temperatures from falling significantly below 0°C, in the vicinity of large bodies of water?
What is the temperature of ice right after it is formed by freezing water?
In very humid climates where there are numerous bodies of water, such as in Florida, it is unusual for temperatures to rise above about 35°C (95°F). In deserts, however, temperatures can rise far above this. Explain how the evaporation of water helps limit high temperatures in humid climates.
In winters, it is often warmer in San Francisco than in nearby Sacramento, 150 km inland. In summers, it is nearly always hotter in Sacramento. Explain how the bodies of water surrounding San Francisco moderate its extreme temperatures.
Putting a lid on a boiling pot greatly reduces the heat transfer necessary to keep it boiling. Explain why.
In a physics classroom demonstration, an instructor inflates a balloon by mouth and then cools it in liquid nitrogen. When cold, the shrunken balloon has a small amount of light blue liquid in it, as well as some snow-like crystals. As it warms up, the liquid boils, and part of the crystals
Freeze-dried foods have been dehydrated in a vacuum. During the process, the food freezes and must be heated to facilitate dehydration. Explain both how the vacuum speeds up dehydration and why the food freezes as a result.
When still air cools by radiating at night, it is unusual for temperatures to fall below the dew point. Explain why.
What are the main methods of heat transfer from the hot core of Earth to its surface? From Earth’s surface to outer space?
Figure 14.30 shows a cut-away drawing of a thermos bottle (also known as a Dewar flask), which is a device designed specifically to slow down all forms of heat transfer. Explain the functions of the various parts, such as the vacuum, the silvering of the walls, the thin-walled long glass neck, the
When our bodies get too warm, they respond by sweating and increasing blood circulation to the surface to transfer thermal energy away from the core. What effect will this have on a person in a 40.0°C hot tub?
Some electric stoves have a flat ceramic surface with heating elements hidden beneath. A pot placed over a heating element will be heated, while it is safe to touch the surface only a few centimeters away. Why is ceramic, with a conductivity less than that of a metal but greater than that of a good
Loose-fitting white clothing covering most of the body is ideal for desert dwellers, both in the hot Sun and during cold evenings. Explain how such clothing is advantageous during both day and night.
One way to make a fireplace more energy efficient is to have an external air supply for the combustion of its fuel. Another is to have room air circulate around the outside of the fire box and back into the room. Detail the methods of heat transfer involved in each.
On cold, clear nights horses will sleep under the cover of large trees. How does this help them keep warm?
When watching a daytime circus in a large, dark-colored tent, you sense significant heat transfer from the tent. Explain why this occurs.
Satellites designed to observe the radiation from cold (3 K) dark space have sensors that are shaded from the Sun, Earth, and Moon and that are cooled to very low temperatures. Why must the sensors be at low temperature?
Why are cloudy nights generally warmer than clear ones?
Why are thermometers that are used in weather stations shielded from the sunshine? What does a thermometer measure if it is shielded from the sunshine and also if it is not?
On average, would Earth be warmer or cooler without the atmosphere? Explain your answer.
Describe the photo of the tea kettle at the beginning of this section in terms of heat transfer, work done, and internal energy. How is heat being transferred? What is the work done and what is doing it? How does the kettle maintain its internal energy?
The first law of thermodynamics and the conservation of energy, as discussed in Conservation of Energy, are clearly related. How do they differ in the types of energy considered?
Heat transfer Q and work done W are always energy in transit, whereas internal energy U is energy stored in a system. Give an example of each type of energy, and state specifically how it is either in transit or resides in a system.
How do heat transfer and internal energy differ? In particular, which can be stored as such in a system and which cannot?
If you run down some stairs and stop, what happens to your kinetic energy and your initial gravitational potential energy?
Give an explanation of how food energy (calories) can be viewed as molecular potential energy (consistent with the atomic and molecular definition of internal energy).
Identify the type of energy transferred to your body in each of the following as either internal energy, heat transfer, or doing work: (a) Basking in sunlight; (b) Eating food; (c) Riding an elevator to a higher floor.
One method of converting heat transfer into doing work is for heat transfer into a gas to take place, which expands, doing work on a piston, as shown in the figure below.(a) Is the heat transfer converted directly to work in an isobaric process, or does it go through another form first? Explain
A great deal of effort, time, and money has been spent in the quest for the so-called perpetual motion machine, which is defined as a hypothetical machine that operates or produces useful work indefinitely and/or a hypothetical machine that produces more work or energy than it consumes. Explain, in
We ordinarily say that ΔU = 0 for an isothermal process. Does this assume no phase change takes place? Explain your answer.
Which cyclical process represented by the two closed loops, ABCFA and ABDEA, on the PV diagram in the figure below produces the greatest net work? Is that process also the one with the smallest work input required to return it to point A? Explain your responses. PA A FE B C D V
The temperature of a rapidly expanding gas decreases. Explain why in terms of the first law of thermodynamics.
A real process may be nearly adiabatic if it occurs over a very short time. How does the short time span help the process to be adiabatic?
It is unlikely that a process can be isothermal unless it is a very slow process. Explain why. Is the same true for isobaric and isochoric processes? Explain your answer.
Imagine you are driving a car up Pike’s Peak in Colorado. To raise a car weighing 1000 kilograms a distance of 100 meters would require about a million joules. You could raise a car 12.5 kilometers with the energy in a gallon of gas. Driving up Pike's Peak (a mere 3000-meter climb) should consume
Is a temperature difference necessary to operate a heat engine? State why or why not.
Definitions of efficiency vary depending on how energy is being converted. Compare the definitions of efficiency for the human body and heat engines. How does the definition of efficiency in each relate to the type of energy being converted into doing work?
Why—other than the fact that the second law of thermodynamics says reversible engines are the most efficient—should heat engines employing reversible processes be more efficient than those employing irreversible processes? Consider that dissipative mechanisms are one cause of irreversibility.
Think about the drinking bird at the beginning of this section (Figure 15.20). Although the bird enjoys the theoretical maximum efficiency possible, if left to its own devices over time, the bird will cease “drinking.” What are some of the dissipative processes that might cause the bird’s
Can improved engineering and materials be employed in heat engines to reduce heat transfer into the environment? Can they eliminate heat transfer into the environment entirely?
Does the second law of thermodynamics alter the conservation of energy principle?
Explain why heat pumps do not work as well in very cold climates as they do in milder ones. Is the same true of refrigerators?
In some Northern European nations, homes are being built without heating systems of any type. They are very well insulated and are kept warm by the body heat of the residents. However, when the residents are not at home, it is still warm in these houses. What is a possible explanation?
Why do refrigerators, air conditioners, and heat for cycles pumps operate most cost-effectively with a small difference between Th, and Tc? (Note that the temperatures of the cycle employed are crucial to its COP.)
Grocery store managers contend that there is less total energy consumption in the summer if the store is kept at a low temperature. Make arguments to support or refute this claim, taking into account that there are numerous refrigerators and freezers in the store.
A woman shuts her summer cottage up in September and returns in June. No one has entered the cottage in the meantime. Explain what she is likely to find, in terms of the second law of thermodynamics.
Consider a system with a certain energy content, from which we wish to extract as much work as possible. Should the system’s entropy be high or low? Is this orderly or disorderly? Structured or uniform? Explain briefly.
Does a gas become more orderly when it liquefies? Does its entropy change? If so, does the entropy increase or decrease? Explain your answer.
Explain how water’s entropy can decrease when it freezes without violating the second law of thermodynamics. Specifically, explain what happens to the entropy of its surroundings.
Is a uniform-temperature gas more or less orderly than one with several different temperatures? Which is more structured? In which can heat transfer result in work done without heat transfer from another system?
Give an example of a spontaneous process in which a system becomes less ordered and energy becomes less available to do work. What happens to the system’s entropy in this process?
What is the change in entropy in an adiabatic process? Does this imply that adiabatic processes are reversible? Can a process be precisely adiabatic for a macroscopic system?
Does the entropy of a star increase or decrease as it radiates? Does the entropy of the space into which it radiates (which has a temperature of about 3 K) increase or decrease? What does this do to the entropy of the universe?
Describe a system in which elastic potential energy is stored.
(a) If frequency is not constant for some oscillation, can the oscillation be simple harmonic motion?(b) Can you think of any examples of harmonic motion where the frequency may depend on the amplitude?
Give an example of a simple harmonic oscillator, specifically noting how its frequency is independent of amplitude.
As you pass a freight truck with a trailer on a highway, you notice that its trailer is bouncing up and down slowly. Is it more likely that the trailer is heavily loaded or nearly empty? Explain your answer.
Some people modify cars to be much closer to the ground than when manufactured. Should they install stiffer springs? Explain your answer.
Pendulum clocks are made to run at the correct rate by adjusting the pendulum’s length. Suppose you move from one city to another where the acceleration due to gravity is slightly greater, taking your pendulum clock with you, will you have to lengthen or shorten the pendulum to keep the correct
Explain in terms of energy how dissipative forces such as friction reduce the amplitude of a harmonic oscillator. Also explain how a driving mechanism can compensate. (A pendulum clock is such a system.)
Give an example of a damped harmonic oscillator. (They are more common than undamped or simple harmonic oscillators.)
How would a car bounce after a bump under each of these conditions?• Overdamping • Underdamping • Critical damping
Most harmonic oscillators are damped and, if undriven, eventually come to a stop. How is this observation related to the second law of thermodynamics?
Why are soldiers in general ordered to “route step” (walk out of step) across a bridge?
Give one example of a transverse wave and another of a longitudinal wave, being careful to note the relative directions of the disturbance and wave propagation in each.
Speakers in stereo systems have two color-coded terminals to indicate how to hook up the wires. If the wires are reversed, the speaker moves in a direction opposite that of a properly connected speaker. Explain why it is important to have both speakers connected the same way.
Two identical waves undergo pure constructive interference. Is the resultant intensity twice that of the individual waves? Explain your answer.
Circular water waves decrease in amplitude as they move away from where a rock is dropped. Explain why.
Liquids and solids expand with increasing temperature, because the kinetic energy of a body’s atoms and molecules increases. Explain why some materials shrink with increasing temperature.
The vapor pressure of water at 40.0°C is 7.34 x 103 N/m2. Using the ideal gas law, calculate the density of water vapor in g/m3 that creates a partial pressure equal to this vapor pressure. The result should be the same as the saturation vapor density at that temperature (51.1 g/m3).
Find out the human population of Earth. Is there a mole of people inhabiting Earth? If the average mass of a person is 60 kg, calculate the mass of a mole of people. How does the mass of a mole of people compare with the mass of Earth?
Does the temperature of a body depend on its size?
"Room temperature" is generally defined to be 25°C.(a) What is room temperature in °F?(b) What is it in K?StrategyTo answer these questions, all we need to do is choose the correct conversion equations and plug in the known values.
What does it mean to say that two systems are in thermal equilibrium?
Two blocks, A and B, are made of the same material. Block A has dimensions l x w x h = L x 2L x L and Block B has dimensions 2L x 2L x 2L. If the temperature changes, what is(a) The change in the volume of the two blocks,(b) The change in the cross-sectional area l × w, and(c) The change in the
Frost damage to most plants occurs at temperatures of 28.0°F or lower. What is this temperature on the Kelvin scale?
The Reaumur scale is a temperature scale that was used widely in Europe in the 18th and 19th centuries. On the Reaumur temperature scale, the freezing point of water is 0°R and the boiling temperature is 80°R. If "room temperature" is 25°C on the Celsius scale, what is it on the Reaumur
Give an example of a physical property that varies with temperature and describe how it is used to measure temperature.
The main span of San Francisco's Golden Gate Bridge is 1275 m long at its coldest. The bridge is exposed to temperatures ranging from -15°C to 40°C. What is its change in length between these temperatures? Assume that the bridge is made entirely of steel.StrategyUse the equation for linear
The active ingredient in a Tylenol pill is 325 mg of acetaminophen (C8H9 NO2). Find the number of active molecules of acetaminophen in a single pill.
To conserve energy, room temperatures are kept at 68.0°F in the winter and 78.0°F in the summer. What are these temperatures on the Celsius scale?
When a cold alcohol thermometer is placed in a hot liquid, the column of alcohol goes down slightly before going up. Explain why.
The density of air at standard conditions (P = 1 atm and T = 20°C) is 1.28 kg/m3. At what pressure is the density 0.64 kg/m3 if the temperature and number of molecules are kept constant?
A tungsten light bulb filament may operate at 2900 K. What is its Fahrenheit temperature? What is this on the Celsius scale?
If you add boiling water to a cup at room temperature, what would you expect the final equilibrium temperature of the unit to be? You will need to include the surroundings as part of the system. Consider the zeroth law of thermodynamics.
What pressure would be created in the gasoline tank considered in Example 13.4, if the gasoline increases in temperature from 15.0°C to 35.0°C without being allowed to expand? Assume that the bulk modulus B for gasoline is 1.00 x 109 N/m2. (For more on bulk modulus, see Elasticity: Stress and
Liquids and solids have densities about 1000 times greater than gases. Explain how this implies that the distances between atoms and molecules in gases are about 10 times greater than the size of their atoms and molecules.
The surface temperature of the Sun is about 5750 K. What is this temperature on the Fahrenheit scale?
Suppose your bicycle tire is fully inflated, with an absolute pressure of 7.00 x 105 Pa (a gauge pressure of just under 90.0 lb/in2) at a temperature of 18.0°C. What is the pressure after its temperature has risen to 35.0°C? Assume that there are no appreciable leaks or changes in
Thermal stresses caused by uneven cooling can easily break glass cookware. Explain why Pyrex®, a glass with a small coefficient of linear expansion, is less susceptible.
If you consider a very small object such as a grain of pollen, in a gas, then the number of atoms and molecules striking its surface would also be relatively small. Would the grain of pollen experience any fluctuations in pressure due to statistical fluctuations in the number of gas atoms and
One of the hottest temperatures ever recorded on the surface of Earth was 134°F in Death Valley, CA. What is this temperature in Celsius degrees? What is this temperature in Kelvin?
Water expands significantly when it freezes: a volume increase of about 9% occurs. As a result of this expansion and because of the formation and growth of crystals as water freezes, anywhere from 10% to 30% of biological cells are burst when animal or plant material is frozen. Discuss the
Explain why a cup of water (or soda) with ice cubes stays at 0°C, even on a hot summer day.
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